The present invention relates to a laser annealing method and a laser annealing apparatus suitable for irradiating an amorphous or polycrystalline semiconductor film formed on an insulating substrate with a laser beam so as to improve the film quality or enlarge or mono-crystallize crystal grains.
At present, a flat panel type display unit using a display panel such as a liquid crystal panel or an organic EL panel is developed and put or being put into practical use. The display panel in such a display unit has a large number of pixels in its display region. The pixels are arrayed two-dimensionally (in a matrix) out of thin film transistors (TFTs) formed of an amorphous silicon film or a polycrystalline silicon film formed on a substrate of glass, fused quartz or the like. An image is formed by switching on/off the thin film transistors corresponding to the pixels. The thin film transistors of the pixels are driven and switched by a driver circuit provided outside the display region.
If the driver circuit for driving the thin film transistors of the pixels can be formed together on the substrate of the display panel, it can be expected to dramatically reduce the manufacturing cost and improve the reliability. However, due to the poor crystallinity of the silicon film forming an active layer of each thin film transistor, the performance of the thin film transistor represented by mobility is so low that it is difficult to produce a circuit required to have a high speed and a high function, such as the aforementioned driver circuit.
High-mobility thin film transistors are required to produce such a high-speed and high-function circuit. To this end, it is necessary to improve the crystallinity of the silicon thin film. Excimer laser annealing has got a lot of attention as a method for improving the crystallinity. According to this method, an amorphous silicon film formed on an insulating substrate of glass or the like is irradiated with an excimer laser so that the amorphous silicon film is transformed into a polycrystalline silicon film. Thus, the mobility is improved. However, the polycrystalline film obtained by the excimer laser irradiation has a crystal gain size of about several tens of nanometers to several hundreds of nanometers, which is insufficient in terms of performance to be applied to a driver circuit for driving a liquid crystal panel, or the like.
As a solution to this problem, Patent Document 1 discloses a method in which a temporally modulated in amplitude continuous wave laser beam is condensed linearly so as to perform irradiation while scanning at a high speed. Thus, crystal grains are grown laterally with respect to the scanning direction so as to increase the crystal grain size. According to this method, the whole surface of a substrate is made polycrystalline due to excimer laser annealing, and only a region where a driver circuit should be formed is then scanned with a laser beam in a direction following a current path (drain-source direction, also referred to as “channel”) of a transistor to be formed, so as to grow crystal grains. As a result, any grain boundary crossing the current path is prevented from existing, so that the mobility is improved on a large scale.
Patent Document 2 discloses a laser annealing technique in which the longitudinal direction of a laser beam spot is made parallel to the longitudinal direction of a region to be irradiated therewith so that the irradiation efficiency of the laser beam is enhanced.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-124136    [Patent Document 2] Japanese Patent Laid-Open No. 209069/1998    [Patent Document 3] Japanese Patent Laid-Open No. 352419/1999